1. Field of the Invention
The invention relates to an apparatus for recording a digital information signal in information tracks of a record carrier. Such apparatus is described inter alia in U.S. Pat. No. 4,511,933 (PHN 10.443) and published European Patent Application No. 250,049 (PHN 11.790), both assigned to the present assignee.
2. Description of the Related Art
Said U.S. Patent describes apparatus for recording a digital information signal in information tracks of a record carrier and, prior to recording, converting successive information words of such digital signal into channel words of a selected channel code. The selected channel code is such that that at least two channel words of mutually opposite disparity are available for each of a plurality of information words of the applied digital information signal. The apparatus comprises:
an input terminal for receiving the information words, PA1 an encoding device having a first input coupled to the input terminal, a second input for receiving a control signal, and an output; which encoding device is adapted to select, depending on the applied information word and the control signal, a channel word from the channel words which are available for the applied information word and to supply the selected channel word to the output of the apparatus, PA1 a control-signal unit, having an output coupled to the second input of the encoding device and having an input for connection to a signal generator, and PA1 an output terminal coupled to the output of the encoding device to supply the channel words. PA1 read means for reading the digital signal from the information tracks on the record carrier, having an output coupled to an input of PA1 an integrating amplifier having an input coupled to the output of the read means, and PA1 a decoder unit coupled to the output of said amplifier and which decodes the channel words into information words which are supplied to an output terminal.
For inserting a second signal in the data stream of channel words, which second signal has a comparatively small low-frequency content relative to the recorded information signal, the control-signal unit is adapted to generate a control signal and the encoding device is adapted to make a choice from the channel words available for each information word depending on said control signal, in such a way that the average value of the recorded digital signal varies substantially in conformity with the variation of the second signal of comparatively low frequency. In this apparatus the second signal is a pilot signal comprising a carrier wave of comparatively low frequency relative to the recorded information signal, which pilot signal is intended to be recorded in the information tracks on the record carrier as a tracking signal to provide information about the relative position of a read element with respect to the information track to be scanned, i.e. the position in a direction transverse to the track direction. When a specific information track is read by the read element the comparatively low frequency of the tracking signals (varying d.c. content) gives rise to comparatively strong crosstalk of the tracking signals recorded in the two adjacent information tracks. Since the magnitude of the crosstalk between these two adjacent information tracks and the read element depends on the positioning of the read element relative to the information track to be read, it is possible to derive from the magnitude of the crosstalk signals a control signal which is representative of the position of the read element relative to the information track to be read. This control signal can then be used for controlling the position of the read element relative to the information track to be read.
It is to be noted that it is known per se to use tracking signals of comparatively low frequency for the purpose of controlling the position of a read element relative to the information tracks. Examples of the use of such tracking signals to record and read a video signal on a magnetic record-carrier tape can be found in U.S. Pat. Nos. 4,056,832; 4,110,799; and 4,297,733, all assigned to the present assignee. However, in all these systems the tracking signal to be recorded is an additional signal which is added to the information signal to be recorded.
In contradiction to this, the apparatus of the above-described U.S. Pat. No. 4,511,933 does not add an additional tracking signal to the digital signal to be recorded, but instead the coding is adapted in such a way that the digital sum value of the recorded digital signal is representative of the tracking signal. However, the recorded signal remains a fully digital signal, i.e. a purely binary signal when a binary code is used and a purely ternary signal in the case of a ternary code.
The known apparatus preferably converts information words of five data bits into channel words of seven data bits. The conversion of information words into channel words is effected in such a way that for one information word two channel words of equal but opposite disparity, for example .+-.2, are available.
The known apparatus has the disadvantage that the second signal is in the form of a pilot signal having only one amplitude and one waveform, namely a square waveform.